Hi everybody!
I posted actually the same problem yet (https://forums.gentoo.org/viewtopic.php?p=1504321&highlight=&sid=903408085d57153947293376f23f9428#1504321). But I continued trying but I'm no longer sure if the problem is really yaboot or another.
Since over 2 months I'm trying to install Gentoo on my new PowerMac G4 DUal 1,25GHz. I never had serious problems with the installations on my Dell notebook, but on the Mac it simply does not work. I follow the handbook strictly and normally everything works fine.
I boot with
G4-SMP video=radeonfb:1280x1024@75
and generally it works fine. Sometimes (!) I get only the following message:

Until the yaboot-configuration there are no problems. Yaboot complains about a too old kernel, but as I've read in other posts, it seems to be normal (??). I umount all mounted partitions, and type reboot, it never worked (see my post here: https://forums.gentoo.org/viewtopic.php?p=1586300&highlight=#1586300). I shut down the mac manually and switch it on again. The only thing I see is a grey screen, nothing else. No boot message, symbol, error message, nothing. I just want finally a working system, otherwise I might re-sell the mac and buy me another pc - but I preferred my mac!! Here are all configurations:
/etc/fstab

As for the kernel: previously I compiled it myself, then I was afear it might have been an error of mine in compilation and so I used the genkernel config. I know, it's a 2.6.5 and no 2.6.7. I'm not sure if there are greater changes that might create problems. I thought even to change to 2.6.8 but I found persons who got work 2.6.7-kernel as well.
Any comment and help is very appreciated, if you need further information ask and I will try to post it. I re-typed everything because I have no option to copy and paste or something like that, so I hope there are no errors. Shouldn't be :)
Thanks!

I have a similar problem. Around every 10th time the boot stops at exactly the same spot, after the firmware output but before booting the kernel. Seems to be pretty randomly. When I reset my machine, it often happens again but never the third time (until now).
Normally I don't reboot my Mac very often so it's not so bad for me. Seems to be a bug in yaboot or the kernel.